Stars
and Navigation
Imagine waking up in the middle of nowhere, all alone. How would you know
where you were?

If
it were nighttime and the sky was clear, you could look up at the stars.
If you could measure precisely what angle various stars were from you
at precise hours, you could calculate your position. That is what sailors
did for many centuries, using a special tool called a sextant.

Many
birds that migrate at nighttime use the stars for navigation, too. They
don't use a sextant. They simply migrate toward or away from the North
Star.

GPS:
Navigating Made Easier
The problem with depending on the stars to set a direction or find out
where you are is that stars are only visible at nighttime, and only when
clouds don't cover them up. That's why the Global Positioning
System(GPS) was started. Instead of stars,
this system uses satellites—"human-made stars." GPS is
a worldwide radio-navigation system based on 24 satellites and their five
ground stations. Each of the satellites sends out a unique radio signal.

A
GPS receiver (click the photo to enlarge) works like
a radio--buy instead of getting signals from radio stations, it gets its
signals from the satellites. As long as it can get signals from at least
three or four satellites, a GPS receiver can calculate precisely where
on Earth it is. The calculations are based on the time it takes for signals
from the satellites to reach the receiver. GPS can be used to figure out
where you are no matter where on the planet you happen to be, no matter
what time of day, in any weather.

Moving
Right Along: GPS Helps!A
GPS receiver can do more than calculate where it is. It can also keep
a constant record of how fast it is moving and what its altitude is. For
example, ultralight pilots rely on their GPS receivers to tell them how
far and how long they must fly before they get to their stopover sites.This
lets them know if the birds can make it without tiring and needing to
land. If a crane gets tired and drops out, the pilot can radio the location
to ground trackers, who can then locate the "missing" bird.
Manatee and eagle scientists also use GPS tracking to follow the migrations
of those species.

Try
this! Discussion and Journaling

Look at
the longitude and latitude on the GPS above. Where was this GPS receiver
photo taken?

Since
the Ultralight airplanes flying with the cranes have a GPS receiver,
why do you think they also have an altimeter
and a speedometer?

Would
the GPS receiver show the plane's ground speed or air speed? Why?

Now GPS
receivers are on airplanes, buses, and even many cars. What are some
situations in which a GPS could be helpful for travelers?